The present invention is directed to magnetic sensor wires. More particularly, the invention relates to a copper alloy having a magnetic second phase which generates a voltage pulse when passed through a magnetic field.
U.S. Pat. No. 3,820,090 to Wiegand discloses a bistable ferromagnetic wire. The wire has a core with low magnetic coercivity and an outer shell having a higher coercivity. The core and shell are magnetized in either the same or opposite longitudinal directions. Exposure to a sufficiently high magnetic field will induce a flux reversal in either the core alone or the core and shell. The shell having higher magnetic coercivity requires a stronger magnetic field to change direction. The direction of magnetization of the core can be changed without affecting the polarity of the shell. By proper manipulation of an external magnetic field, the direction of magnetization of the core can be switched to the same or opposite that of the shell.
The polarity switching occurs abruptly and corresponds to a point of discontinuity in the hysteresis loop of the sensor wire. The discontinuity is known to those skilled in the art as the Barkhausen effect. The Barkhausen effect generates a significant voltage pulse which may exceed 2 volts and is detectable by an external receiver.
Wires such as those disclosed in U.S. Pat. No. 3,820,090 have many applications. The speed of a rotating shaft may be determined without a detector physically contacting the shaft. A bistable wire is mounted on the shaft at one point along its circumference. As the shaft rotates, the wire passes through an external magnetic field generating a voltage pulse each time the wire enters the field. The voltage pulses are be counted by an external detector.
Other applications of the sensor wires include use as a magnetic key. A series of sensor wires are mounted on a nonmagnetic backing plate. By varying the direction of polarity of the core and shell, a unique voltage pattern is generated when the wire passes through a magnetic sensor. A detector is programmed to recognize that specific voltage pattern.
Sensor wires are also used in retail stores to deter shoplifting. The shift in the polarity of the core is detected as a voltage pulse triggering an alarm. Removal of the sensor by a sales clerk prevents triggering the alarm on exit.
Several means to form a bistable magnetic sensor wire have been disclosed. U.S. Pat. No. 3,820,090 discloses controlled heating and cooling of a precipitation hardenable magnetic wire such that the shell is hardened to a greater extent than the core. U.S. Pat. No. 3,892,118 to Wiegand, discloses increasing the magnetic coercivity of the outer shell by mechanical working. The wire is twisted while under tension.
Another way to form magnetically hard and soft regions is disclosed in U.S. Pat. No. 4,913,750 to Kakuno et al. A magnetic wire is drawn through a die to produce a hardened wire. Portions of the wire are then annealed to produce magnetically soft regions. U.S. Pat. No. 4,950,550 to Radeloff et al discloses cladding a magnetically hard material such as cobalt-vanadium-iron around a magnetically soft core such as nickel-iron.
The properties affecting the performance of the sensor wire include magnetic saturation, coercivity, the Barkhausen effect and the Curie temperature. Magnetic saturation is the level of magnetization after which additional increases in magnetic field strength do not produce additional magnetization. The larger the peak magnetization, the more pronounced the change when the polarity reverses and the larger the corresponding voltage pulse.
Coercivity is the strength of an external magnetic field which must be applied to cause the direction of magnetization to reverse. To prevent inadvertent changes in polarity, the shell of the sensor wire must have a higher coercivity than the core.
The Barkhausen effect is a discontinuity in the magnetic hysteresis loop of the sensor wire corresponding to polarity switching of the core or the shell. The more distinct the interface between core and shell, the more pronounced the effect and the larger the generated pulse.
The Curie temperature marks the transition between ferromagnetism and paramagnetism and identifies the maximum operating temperature of the sensor device.
One alloy used for sensor wires is Vicalloy (nominal composition 52% by weight cobalt, 10% vanadium and the remainder iron) as disclosed in U.S. Pat. No. 4,247,601 to Wiegand. A magnetically hard shell is formed by work hardening. A length of Vicalloy alloy wire is twisted while under tension, generating a shell with a higher magnetic coercivity than the core. The bistable Vicalloy alloy wire is characterized by a saturation magnetism of about 170 EMU/g, a coercivity of 30-60 oersteds and a Curie temperature of about 500.degree. C. The material produces a voltage pulse in the range of from about 0.5 to about 2 volts. The alloy is expensive and contains strategically important cobalt and vanadium which are at times in short supply. Additionally, the work hardened wire is brittle and difficult to work.
Accordingly, it is an object of the invention to provide a bistable magnetic sensor wire which does not have the limitations of the prior art. It is a feature of the invention that either the core, the shell or both is an alloy with a copper matrix and a magnetic second phase. The wires have a magnetic saturation of about 60 EMU/g and a coercivity of anywhere from about 30 to about 900 oersteds.
It is an advantage of the present invention that the magnetic alloys do not contain strategic materials and are significantly less expensive than prior art sensor wires. The alloys have improved ductility, permitting the fabrication of more precise sensors. The voltage outputs are increased due to a sharp magnetic gradient.
The above stated objects, features and advantages will become more clear from the specification and drawings which follow.